When a rectangle’s height is larger than its width, you’re looking at a portrait format. This format naturally encourages us to scan a composition from top to bottom. It’s commonly seen in printed material — magazines, posters, flyers, etc. — as well as on mobile devices. 

People have a tendency to keep their phone in portrait orientation even when viewing content that could better be viewed by turning their phone sideways (such as widescreen video).^[1] That’s important to keep in mind when designing for mobile devices.